The sounds of the letters ژ, ,گ, چ, پ have always been part of Persian, but I think their current written forms were added after Arabs invaded Iran 1400 years ago.
I read somewhere that before the Arab conquest of Iran, an advanced form of alphabet was used by Persians which is today known as "Din Dabereh". It has 48 letters including 14 vowels and 34 consonants.
After the Arab conquest, the Arabic script was forced on Persians. Although the Arabic script was not capable of recording all sounds of the Persian language even after addition of a few letters like ژ, ,گ, چ, پ, it became the official script for writing Persian.

Thanks for your reply Ben422,
What I'm really wondering is how long after the introduction of the Arabic script did the Persians add to the Arabic those letters. I'm thinking that the script originally was only used for the Quran and religious texts in the Arabic language. So it would have been after the Arabic script replaced the "Din Dabereh" that there would have been a need to add more letters.

hello there ......
thank you for your question ...Do you know who wrote the Arabic grammar ?
let me tell u .... 4 iranian persons did it .... all of the grammatical rules were written by them ... but the hint is arabic language was accepted by iranian people as their religious language and they really adored that because of this reason , nothing else ....
these letters were added to make different iranian and arabic alphabets ..

What I'm really wondering is how long after the introduction of the Arabic script did the Persians add to the Arabic those letters.

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That's really an interesting question. Unfortunately, I don't have much information on the Persian literature history, but according to the following page, the Persians added those letters in the 14th century (Note that the Arab invasion of Iran took place in 637 AD):

Did you know when the letters «گ‌چ‌پ‌ژ» first entered the Persian language?

Before the 8th century AH, Iranians had made no changes in the Arabic literature about the four special Persian letters not found in Arabic, i.e. ز, چ, پ and گ, and it was after the 8th century AH that the current spelling, i.e using 3 dots for چ, پ and ژ and two sarkeshes for گ began to come into widespread use.

A very interesting question! I have wondered about this question as well as the addition of further letters which form the Urdu alphabet.

From what I have been able to find so far is that the Persian language began to be written in Arabic alphabet in 642AD. I have no doubt in my mind (although it does n't matter to me) that the person/s coming up with p/ch/g and Z were indiginous individuals who, having mastered the Arabic language and its alphabet applied their innovations to Persian. I don't think it is just a matter of co-incidence that p/b, ch/j, k/g and z/Z shapes are identical. The first three are linguistically connected with each other (voiceless/voiced) and credit is due to the innovator/s. Z is strictly speaking linked to "sh" and not z, but, hey, no one is perfect!

You are right Qureshpor, but then the Arabic letter sheen /sh/ already has three dots. The zay /z/ is also a sibilant like /zh/ and /sh/ so I suppose they made do with it for that.

But I've now realised there is another question on these lines! From talking about Persian language written with the Arabic script such as the Shahmaneh by Firdausi... the question is this : What letters in the Arabic script were generally applied for those sounds for which later the new letters would make. For example, a word such as "Parsi"; was it written in the Arabic script as Barsi or Farsi? Or "magi"; was it written maki, maji, maghi, maqi?

You are right Qureshpor, but then the Arabic letter sheen /sh/ already has three dots. The zay /z/ is also a sibilant like /zh/ and /sh/ so I suppose they made do with it for that.

But I've now realised there is another question on these lines! From talking about Persian language written with the Arabic script such as the Shahmaneh by Firdausi... the question is this : What letters in the Arabic script were generally applied for those sounds for which later the new letters would make. For example, a word such as "Parsi"; was it written in the Arabic script as Barsi or Farsi? Or "magi"; was it written maki, maji, maghi, maqi?

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OK, Let's take one letter at a time.

p > b
e.g. baadshaah. (King) This is the normal word for king in Urdu.

P > f
e.g. Khaliij-i-faars (The Persian Gulf)
e.g. fiil (elephant). This word occurs in the Qur'an too. (The original was piil)

ch>j

It is possible that I might be wrong about this but the word for a chick was "chuuchah" (still is in Afghan Persian and Punjabi) which is "chuuzah" in Urdu but "juujeh" in the modern Farsi language.

g>j

Giilaan (place name) > Jiilaan (as in Abdul Qaadir Jiilaanii)

Z> j (? ) kaZdum (scorption), kaj = bent

By the way Mr.St.Francis, g, originally was a kaaf with three dots above the slant. Only later did the kaaf acquire an additional slant to give it the gaaf identity.

Yes, I did see a kaf with three dots in my own search for the answers, but I think it was from Morocco rather than Persia so I wasn't sure it was related but rather coincidental. However, the slant on top of the kaf originally being three dots does make sense because the other Persian letters have three dots.